https://wccftech.com/the-witcher-3-custom-story-quests/

The Witcher 3 received a lot of mods since its release, but very few actually introduce new content to the game. Things, however, are about the change, as it is now possible to create custom story quests thanks to a new set of modding tools.

The Radish Modding Tools finally allow the creation of custom story quests in The Witcher 3. These tools are not easy to use, so it may take some time to see more complex quests coming to the game.